Clinical Fellow in Anaesthesia, Harvard Medical School; Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Anuja Antony, MD, MPH
Assistant Professor, University of Illinois at Chicago; Clinical Research Fellow, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
William G. Austen Jr, MD
Chief, Division of Plastic and Reconstructive Surgery, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
Arna Banerjee, MD
Assistant Professor of Anesthesiology and Surgery, Vanderbilt University Medical Center; Medical Co-Director, Surgical ICU, VA Tennessee Valley Healthcare System, Nashville, Tennessee
Sergio D. Bergese, MD
Director of Neuroanesthesia, Department of Anesthesiology, The Ohio State University, Columbus, Ohio
Arnold Berry, MD, MPH
Professor of Anesthesiology, Emory University School of Medicine, Atlanta, Georgia
John A. Carter, MBBS, FRCA
Consultant in Anaesthesia and Critical Care Medicine, Department of Anaesthesia, Frenchay Hospital, Bristol, UK
Jennifer A. Chatburn, MD
Clinical Fellow in Anesthesia, Harvard Medical School; Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Marianna P. Crowley, MD
Assistant Professor, Harvard Medical School; Anesthetist, Massachusetts General Hospital, Boston, Massachusetts
Paul D. Davis, BSc
Principal Physicist, Department of Clinical Physics and Bioengineering, Southern General Hospital, Glasgow, UK
Harold J. DeMonaco, MS
Director, Innovation Support Center, Massachusetts General Hospital, Boston, Massachusetts
Ali Diba, BM, FRCA
Consultant Anaesthetist, Anaesthetic Department, Queen Victoria Hospital NHS Foundation Trust, East Grinstead, UK
Richard P. Dutton, MD, MBA
Professor of Anesthesiology, University of Maryland Medical Center, Baltimore, Maryland
Jane Easdown, MD
Associate Professor of Anesthesiology and Associate Residency Director, Vanderbilt University Medical Center, Nashville, Tennessee
Jesse Ehrenfeld, MD
Assistant Professor of Anaesthesia, Harvard Medical School; Director of Anesthesia Informatics Fellowship, Director of Anesthesia Clinical Research Center, Department of Anesthesia, Critical Care, and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Stephanie Ennis, NP
Nurse Practitioner, Cardiology Service, Massachusetts General Hospital, Boston, Massachusetts
Roy K. Esaki, MD, MS
Resident, Department of Anesthesia, Stanford University School of Medicine, Palo Alto, California
Jeffrey M. Feldman, MD
Division Chief, General Anesthesiology, Children’s Hospital of Philadelphia, Philadelphia, Pennsylvania
Gayle Fishman, BSN, MBA
Vice President of Clinical Services, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts
Michael G. Fitzsimons, MD
Assistant Professor of Anaesthesia, Harvard Medical School; Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Consultant in Anaesthesia and Intensive Care Medicine, North Bristol NHS Trust, Bristol, UK
Vanessa Henke, MD
Department of Anaesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Robert Holzman, MD
Senior Associate in Anesthesiology, Children’s Hospital Boston; Associate Professor of Anaesthesia, Harvard Medical School, Boston, Massachusetts
Yandong Jiang, MD, PHD
Assistant Professor of Anaesthesia, Harvard Medical School; Department of Anesthesia, Critical Care, and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Robert M. Kacmarek, PhD
Professor of Anaesthesia, Harvard Medical School; Director of Respiratory Care, Massachusetts General Hospital, Boston, Massachusetts
Jacob Kaczmarski, MD
Staff Physician, Baptist Hospital of Miami, Miami, Florida
Sachin Kheterpal, MD, MBA
Assistant Professor of Anesthesiology, University of Michigan Medical School, Ann Arbor, Michigan
M. Ellen Kinnealey, BSN
Advanced Infusion Systems Specialist, Massachusetts General Hospital, Boston, Massachusetts
Rebecca Lintner, MD
Assistant Professor of Anesthesiology, Mount Sinai School of Medicine, New York, New York
Thomas E. MacGillivray, MD
Assistant Professor of Surgery, Harvard Medical School; Division of Cardiac Surgery, Massachusetts General Hospital, Boston, Massachusetts
George Mashour, MD, PhD
Director, Division of Neuroanesthesiology, Assistant Professor of Anesthesiology and Neurosurgery, University of Michigan Medical School, Ann Arbor, Michigan
Rafael Montecino, MD
Clinical Assistant Professor of Surgery, Leavenworth VA Medical Center, University of Kansas, Lawrence, Kansas
Beverly Newhouse, MD
Assistant Clinical Professor of Anesthesiology and Critical Care, University of California− San Diego Medical Center, San Diego, California
Jordan L. Newmark, MD
Clinical Fellow in Anaesthesia, Harvard Medical School; Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Michael Oleyar, DO, JD
Michigan State University College of Osteopathic Medicine, East Lansing, Michigan
Eric Pierce, MD, PhD
Assistant Professor, Harvard Medical School; Vice-Chair, Anesthesia Quality Assurance Committee, Massachusetts General Hospital, Boston, Massachusetts
Erika G. Puente, MD
Professor of Anesthesiology, Surgery and Biomedical Informatics
Chair, Department of Anesthesiology, Vanderbilt University School of Medicine
Warren S. Sandberg, MD, PhD
Professor of Anesthesiology, Surgery and Biomedical Informatics
Chair, Department of Anesthesiology, Vanderbilt University School of Medicine, Nashville, Tennessee
F. Jacob Seagull, PhD
Assistant Professor, Division of General Surgery, University of Maryland Medical School, Baltimore, Maryland
Nathaniel M. Sims, MD
Assistant Professor of Anaesthesia, Harvard Medical School; Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Reuben Slater, FANZCA
Staff Anaesthetist, St. Vincent’s Hospital, Melbourne, Australia
Demet Suleymanci, MD
Research Fellow, Department of Anesthesia, Critical Care, and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Sugantha Sundar, MD
Assistant Professor of Anaesthesia, Harvard Medical School, Beth Israel Deaconess Medical Center, Boston, Massachusetts
Richard D. Urman, MD, MBA
Assistant Professor of Anethesia, Harvard Medical School; Director of Procedural Sedation Management, Department of Anesthesiology, Perioperative and Pain Management, Brigham and Women’s Hospital, Boston, Massachusetts
Lisa Warren, MD
Instructor in Anesthesia, Harvard Medical School; Director, Ambulatory and Regional Anesthesia, Department of Anesthesia, Critical Care, and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts
Matthew B. Weinger, MD
Professor of Anesthesiology, Medical Simulation, and Biomedical Informatics, Vanderbilt University Medical Center; Senior Physician Scientist, Geriatric Research Education and Clinical Center, VA Tennessee Valley Healthcare System, Nashville, Tennessee
Zhongcong Xie, MD
Associate Professor of Anaesthesia, Harvard Medical School, Boston, Massachusetts
Zhipeng (David) Xu, MD, PhD
Research Fellow of Anaesthesia, Harvard Medical School, Boston, Massachusetts
Chunbai Zhang, MD, MPH
Chief Resident, Occupational and Environmental Medicine and Epidemiology, Harvard School of Public Health, Boston, Massachusetts
Gilat Zisman, BS
Post-Doctoral Researcher, Department of Anesthesiology, The Ohio State University, Columbus, Ohio
Preface
Medical technology has changed at a rapid pace over the past 30 years and continues to evolve quickly as new devices and techniques change and facilitate the way we practice anesthesiology. For example, a mere 15 years ago, ultrasound was a luxury in anesthesia. Today portable ultrasound has become a de facto standard of care for central venous catheter placement and for regional anesthesia. There are numerous examples of the profusion of such ‘ancillary’ anesthesia equipment, with completely new classes of equipment appearing almost overnight. On the other hand, some aspects of technology –such as the anesthesia machine – seem to be fairly constant. However, a closer examination reveals that this is not really correct as modern equipment only appears to function like its predecessors. Learning to operate, diagnose and troubleshoot all of this equipment competes aggressively with the patient- and disease-oriented components of anesthesiology practice.
Our goal in writing this book was to help clinicians better understand the underlying principles behind the equipment they use on a daily basis. In this firstst edition, we cover all of the equipment used in the operating room from the anesthesia machine to airway devices, physiologic monitors, and equipment used for point-of-care testing. We also included chapters on anesthesia information management systems, alarms, challenges encountered working outside of the operating room, and equipment for use in unusual environments such as a field hospital.
We begin this book with a chapter on simulation in anesthesia. This was deliberate – complexity in anesthesia practice has increased to the point where simulation must play a larger
role in the education of future anesthesiologists, including education about the use of equipment and management of equipment in failure mode. It is increasingly problematic to learn how to use equipment ‘on the fly’ with actual patients.
Recognizing that technology evolves rapidly, we sought to illustrate fundamental principles succinctly, rather than provide a completely comprehensive review of each available device within every category. This book represents the collective wisdom of almost one hundred experts in the fields of anesthesiology, biomedical engineering, and technology. We are grateful to all of our contributors whose efforts, insight, and expertise made this book the most accessible and up-todate work of its kind.
We would like to thank a number of individuals without whom this book would not have come to fruition. They include Dr Elisabeth H. Sandberg, Dr Katharine M. Nicodemus, Dr David C. Ehrenfeld, and Dr Zina Matlyuk-Urman. Additionally, we would like to thank our families and colleagues for their tireless support, and the generations of trainees, from whom we have learned as much as we have taught, for their inspiration. Special thanks to the Elsevier editorial team, especially Natasha Andjelkovic and Bradley McIlwain.
We hope you find this book useful and wish you well in your journey through the world of clinical anesthetic equipment.
Warren S. Sandberg, MD, PhD Vanderbilt University
Richard D. Urman, MD, MBA Harvard University
Jesse M. Ehrenfeld, MD, MPH Harvard University
1
Anesthesia Equipment and Patient Safety
Arna Banerjee, L. Jane Easdown, and Matthew B. Weinger
CHAPTER OUTLINE
Anesthesia Safety: Is It a Model or a Myth? 1
Role of Equipment in Anesthesia Safety 1
The Nature of Errors 3
Use Error Versus Device Failure 5
Anesthesia Safety: Is It a Model or a Myth?
Anesthesia has been touted as being one of the safest specialties in medicine. In 1999, the Institute of Medicine (IOM) published a report on medical errors in U.S. hospitals, which noted that anesthesiology had made substantial improvements in patient safety.1 One impetus for reducing medical errors in the 1970s and the 1980s was the soaring cost of medical malpractice. Anesthesiologists responded by establishing national practice standards for patient monitoring, deliberately analyzing adverse events, improving the safety of anesthesia machines, fostering the widespread adoption of new technologies (e.g., pulse oximetry), improving provider training in crisis event management, and creating an independent foundation whose sole purpose was to advance anesthesia patient safety (the Anesthesia Patient Safety Foundation or APSF). Deaths in anesthesia have decreased from 2 deaths per 10,000 anesthetic procedures in the 1980s to about 1 death per 200,000 to 300,000 in 2000.2 These numbers have been validated by surveys conducted in the Netherlands, France, and Australia.3-5 A 2003 report from the Center for Quality Improvement and Patient Safety of the Agency for Healthcare Research and Quality (AHRQ) found 1369 complications from anesthesia in 1,933,085 patients at risk of 0.71 per 1,000 discharges.6 This rate compares favorably to other rates of hospital complications as shown in Table 1–1 6 Not all anesthesiologists believe that anesthesiology is as safe as these data suggest—even in young, healthy patients. For instance, in 2002, Lagasse published an extensive review of the literature in which he concluded that anesthesiology mortality was still in the range of 1 in 10,000.7 Similar contention has been made by other authors.8 It can be very difficult to separate errors or mishaps in anesthesia from surgical mishaps or patient disease. In studies reviewed by Lagasse, the definitions for death in which anesthesia was “associated,“ “related,”
Current Methods for Training New Users of Anesthesia Equipment 5 Is Conventional Training Enough? 5 Conclusion 8
“contributory,” or “preventable” varied widely as did the time windows for defining the perioperative period (24 hours to 30 days). Many of these studies had small numbers and involved single healthcare sites. An alternative way of understanding patient safety in anesthesia is to study “opportunities for error.” This more probabilistic approach focuses on events and their likelihood of causing patient harm. A key advantage of an event, rather than injury focus, is that data can be collected prospectively and the analysis is less likely to be affected by hindsight or outcome bias. For example, Weinger and colleagues introduced the concept of “nonroutine events” (or NRE) and showed that NRE, which represent any deviation in optimal care, occurred in 25% to 35% of all anesthetics in three different academic medical centers.8 Moreover, an NRE data collection system captured seven times more patient injuries than a traditional anesthesia quality assurance reporting system.9 They concluded that anesthesiology is complex and errors still occur, resulting in poor patient outcomes.
Role of Equipment in Anesthesia Safety
In 1978 Cooper et al applied the critical incident technique first described by Flanagan to understand anesthesia incidents. A critical incident was defined as: a human error or equipment failure that could have led (if not discovered or corrected in time) or did lead to an undesirable outcome, ranging from increased length of stay to death.10 In this study, 139 anesthesiologists were interviewed and 1089 preventable incidents were reported. Seventy incidents were deemed a critical event with a substantial negative outcome. They reported that 30% of critical incidents reported by clinicians were related to equipment problems. Nineteen percent were reported instantly, while 11% were reported retrospectively. Twenty-eight percent of these demonstrated inadequate knowledge or familiarity with specific equipment or use of a relatively new technique or device.
Table 1–1 Patient
Safety Indicators: Comparison Between Medical and Surgical Subspecialties6
OF EQUIPMENT FAILURES
from the 1997 Closed Claims Analysis by Caplan et al)
The American Society of Anesthesiologists (ASA) Closed Claims Project (CCP) was initiated in 1985 to collect information about anesthesia-related adverse outcomes. A total of 8496 closed insurance claims have been collected and analyzed. An analysis in 1997 found that only 2% of the claims were related to equipment issues (Figure 1–1). Death or brain damage occurred in 76% of these cases. Misuse of equipment was judged to have occurred in 75% of the cases and true equipment failure in 24%. Overall, 78% of claims were deemed preventable by appropriate use of monitoring.11 Subsequent studies have called into question the validity of this finding since the reviewers were not blinded to case outcome. Subsequent studies in anesthesia showing similar results are summarized in (Table 1–2).
Similar studies have been conducted in other countries. The Australian Incident Monitoring Study issued results on
2000 critical incidents in 1993. One hundred and seventyseven (9%) were due to equipment problems and of these 107 (60%) were due to failures of the anesthesia gas delivery system.13 The National Reporting and Learning System database from the United Kingdom reported similar results. Of 12,606 incidents reported to the National Patient Safety Agency, 13% were related to equipment failures. Of those incidents, 81% caused little or no harm, 18% produced moderate harm, and only 1.2% resulted in severe harm or death.14
As of May 2009, the ASA Closed Claims Database had 71 claims of a total 2945 claims from 1995 to 2003 due to problems with anesthesiology equipment. Eighteen of these claims were for gas delivery equipment. These included one anesthesia machine problem (unspecified), five vaporizer problems, three ventilator problems, and four breathing circuit problems. There were another five claims involving supplemental oxygen equipment or other devices attached to the patient’s endotracheal tube. There were an additional two claims involving malfunctioning Ambu bags. Most equipment problem claims resulted in temporary or nondisabling injuries (66%). There were 8 (11%) permanent and disabling injuries and 16 (23%) deaths. Payment was made in 52 (73%) of these claims, with a median payment (in 2007 inflation adjusted dollars) of $137,525 (range $2720 to $2,825,750).15 Over time, gas delivery problems appear to be decreasing as a proportion of total claims. These types of incidents represented 3% of all claims in the 1970s, 2% in the 1980s, and 1% in the period 1990 to 2003.16
In Canada, medical device problems reported to the Health Protection Branch were
anesthesia devices.17 While only 2.3% of new
to determine the problems associated
Figure 1–1 Adverse anesthetic outcomes from equipment failures.11
Table 1–2 Studies Examining the Incidence of Human Error in Anesthesia Mishaps12
Authors
Cooper JB et al 1978
Craig and Wilson 1981
Cooper JB et al 1984
Retrospective critical incident reporting
359 14% as a result of failure
Retrospective critical incident reporting 81 12% related to failure
Retrospective critical incident reporting 1089 11% as a result of failure
Also instant reporting 239 19%
Keenan and Boyan 1985 All anesthesia-related cardiac arrests 27
Utting JE 1987
Kumar V et al 1988
Deaths or cerebral injury cases reported to medical defense unit of UK 1501 28% of technique errors
Voluntary QA reporting 129 19% related to failure
Cheney FW et al 1989 ASA closed claims study analysis 869 lawsuits ?
Chopra V et al 1992
Voluntary QA reporting 549 21% related to failure
Caplan RA et al 1997 ASA closed claims study of gas delivery equipment claims 72 24%
Weinger et al 2007
Prospective videotaped anesthetics 407 45% as judged by at least two expert reviewers of the actual videotape; usability or failure was considered to be a contributory factor in a nonroutine anesthesia event
devices were classified as anesthesia devices, these devices produced 8.6% of problem reports and 37.5% of alerts. The percentages of recalls and problem reports were also higher in the anesthesia (10.2%) than in other (4.9%) devices.
Why Is It Important to Know Your Equipment If the Mortality Has Decreased Tenfold in the Last Decade?
Although the most recent gas delivery system closed claim was for an event in 2003, equipment failures have been reported in the literature after that and even in 2008. New equipment continues to be introduced and errors reported. If the goal of the APSF, “no patient will be harmed during anesthesia,” is to be realized, then any error occurring due to equipment misuse or malfunction is unacceptable. Safety can plateau or even diminish without constant effort at improvement. Included in Table 1–3 are recent examples in the literature of equipment problems.
The Nature of Errors
Although anesthesia is deemed safe, there are still reports of poor patient outcomes due to equipment failure or misuse. Detailed literature exists on the types of errors and mistakes that occur during the perioperative period and the complex human and organizational factors which lead to it.10, 22-23 The practice of anesthesiology incorporates many sophisticated types of medical equipment both to deliver therapy and to monitor the effects of anesthesia and surgery on patient physiology. Safe anesthesia requires a comprehensive understanding of equipment both for routine cases and during clinical crisis. The following chapters provide key knowledge about commonly used anesthesia equipment; the goal is to prevent misuse, reduce use errors, facilitate failure detection and recovery, and enable clinicians to help manufacturers to design even better equipment in the future.
The complexity of anesthesia has been compared with that of managing nuclear power plants, military campaigns, or airline flight operations. In fact, the induction and emergence phases of general anesthesia can be compared with the “take off and landing” of an aircraft.
Errors involving disasters in power plants (Chernobyl) or factories (Bhopal) have been examined in great detail.24 Although a single event or person may have been initially implicated in each of these disasters, careful analysis has consistently revealed an array of complex organizational problems and system factors interacted over time to produce the accident. Anesthesiology critical events show similar patterns. However, healthcare errors typically involve only one patient and if the outcome is poor, rarely garner public attention.
Most investigations of patient safety in anesthesia have attributed human error as being responsible for 70% to 80% of critical events. However, this is not a useful statistic. Even most outright equipment failures are inevitably due to human errors—failures of design, installation, maintenance, etc. Moreover, as described above, the clinician who ultimately pushes the wrong button is but one factor in a complex chain of events that can lead to an adverse event. Reason suggested that: “Individual unsafe acts are hard to predict but organizational and contextual factors that give rise to them are both diagnosable and manageable.22”
Errors by individuals do occur and can be characterized further according to Reason. There are slips and lapses that result from inattention during routine management. Mistakes, rulebased or knowledge-based, imply deviation from a plan. An example of a rule-based mistake would be the failure of an anesthesiologist to provide a proper rapid sequence induction to secure a patient’s airway leading to aspiration of stomach contents. Rule-based mistakes may result in misapplication of a rule or not understanding the full rationale for its use. Knowledge-based mistakes occur when an event or experience is outside the experience of the provider. This causes the diagnosis and management to be complicated by poor mental
Table 1–3 Recent Examples in the Literature of Equipment Failure
Title Synopsis
A surprising twist: an unusual failure of a keyed filling device specific for a volatile inhaled anesthetic
Michael F. Keresztury, MD
An insidious failure of an oxygen analyzer
Bryan Harris, MD and Matthew B. Weinger, MD, MS
Aestiva ventilation mode selector switch failures
Dietrich Gravenstein, MD; Harshdeep Wilkhu, MD; Edwin B. Liem, MD; Stuart Tilman, MD; Samsun Lampotang, PhD
APSF newsletter: dear sirs, misplaced valve poses potential hazard (Vanderbilt University Med Ctr)
Two cases were described where keyed filling devices for sevoflurane were inadvertently screwed onto isoflurane bottles. The mishaps were possible because the collars on sevoflurane and isoflurane bottles are mirror images of each other. The particular keyed filling device was designed with a flexible outer sleeve and could be screwed onto the wrong bottle while slightly gouging its soft plastic collar. The keyed filling adapters for sevoflurane and isoflurane could each be manipulated to fit the other’s bottle. A manufacturer (Southmedic, Inc., Barrie, Canada) has modified their keyed filling adapters to prevent this unusual circumstance from recurring.
The authors reported a case of oxygen analyzer malfunction that was diagnosed by the failure of the patient to adequately breathe oxygen as a measure of end-tidal oxygen concentration. Those involved with the care of the patient did not notice a warning icon, compliant with international standards, at the time.
Three cases of previously unreported failures of the Bag-Ventilator Switch in Aestiva/5 anesthesia machines (GE Healthcare/Datex-Ohmeda, Madison, Wis.) were described by the authors. Each failure mode produced a large breathingcircuit leak. Examination of the switches revealed a cracked toggle actuator, residue build-up, and cracked selector switch housing as causes for the failures. When a leak with no visible cause develops, consider advancing the mode selector switch fully to its mechanical limit or consider that the toggle actuator or its anchoring mechanism may have failed.
A problem is in the AGSS (active gas scavenging system) option which produces, when the evacuation hose becomes occluded, sustained airway pressures (PEEP) of up to 40 cm. This condition was exacerbated by high fresh gas flows during mechanical ventilation. The AGSS is designed to have an opening in the bottom of a plastic receiver, providing relief of both positive and negative excess pressures. In what appears to be an assembly error, a negative pressure relief valve (similar to a circle system one-way valve) was installed in this opening (see photos below 1 and 2). This provided relief of excess negative pressure (too much evacuation suction), but no positive pressure relief. This valve is used appropriately in the passive system, but the passive system also has a positive pressure relief in the upper portion of the receiver unit.
2006 18
Bottom view of the active scavenging reservoir without valve (correct configuration).
Bottom view of the active scavenging reservoir with valve in place (incorrect configuration).
modeling and false hypotheses. In this situation the practitioner is prone to have fixation or “cognitive lockup” going down the wrong diagnostic pathway or fixating on one cause to the denial to all other clues. Violations imply a deviation from a standard and must be managed in a social context. An example would be the complete disregard for doing a complete machine check in the morning before starting a case.
Conditions that predispose front-line personnel to make operational errors are called latent failure modes and lead to latent errors. Most hazardous systems are well designed to prevent single point failures; it is the occurrence of multiple unlikely events that culminate in a critical event. Prevention of latent errors (failures of design that lead to the occurrence of errors) is where we need to focus through better communication and teamwork, better design and maintenance of equipment, and planning for coordination of patient care.
Use Error Versus Device Failure
A use error is an “act or omission of an act that results in a different medical device response than intended by the manufacturer or expected by the user.25” Use errors can be subject to budgetary constraints, regulatory demands, and limits of technology. It may become necessary to compromise complexity for simplicity for use in complex environments. Monitoring devices may have an array of alarms to increase vigilance but unfortunately, many alarms are confusing and distracting. Equipment designers often do not interface with users and proper feedback mechanisms often do not exist. A report by the International Electrotechnical Committee (IEC), a body that regulates and standardizes electrical and electronic devices, made the following comment:
“Medical practice is increasingly using medical devices for observation and treatment of patients. Use errors caused by inadequate medical device usability have become an increasing cause for concern. Many of the medical devices developed without applying a usability engineering process are nonintuitive, and difficult to learn and to use. As healthcare evolves, less skilled users, including patients themselves, are now using medical devices, and medical devices are becoming more complicated.25”
What Sorts of Problems Can Occur Directly as a Result of Equipment Failure?
Equipment malfunction may be due to poor equipment design, poor user interface, or poorly designed displays or alarms. Sometimes equipment used in the operating room is used in an other environment—ICU or home care—and is inappropriate for the educational level or pace of use. Included is a table of the most common equipment errors (Table 1–4).
Current Methods for Training New Users of Anesthesia Equipment
The Food and Drug Administration (FDA) regulates the manufacture, distribution, importation, and use of medical devices in the United States. The FDA regulates the “labeling” of all medical devices. Labeling includes specific indications for use, all on-device markings, and all instructions
Table 1–4 Common Equipment Errors
Breathing circuit disconnections
Breathing circuit leaks or defective valves
Breathing circuit misconnection
Breathing circuit control error (e.g., failure to adjust APL valve)
Inadvertent gas low control errors
Gas supply problems
Vaporizer control errors (under-dose/over-dose)
Intravenous drug dose errors (including infusion and syringe pumps)
Intravenous drug/fluid delivery system problems
Ventilator missetting or malfunction
Misuse of monitors
Laryngoscope malfunction
Scavenging system problems
Other (e.g., soda lime exhaustion, sensor failure, blood warmer malfunction, etc.)
for use (including user manuals and quick reference guides). While modern medical devices intended for patients’ use often include well-designed training materials, this is much less common for devices intended for clinicians. In fact, with the recent exception of very high risk devices (e.g., carotid stenting), the FDA does not mandate clinician training before device use.
Conventionally, new anesthesia equipment is introduced through an “in-service” session. An in-service is an educational session during which the company demonstrates the equipment to intended users. These efforts are usually voluntary, superficial, and inadequate because they do not allow the individuals to practice with the device nor do they include an assessment of user understanding or competency. Moreover, in-services typically occur only once with installation and are not repeated for personnel who are away from work or who join the facility thereafter. Device manufacturers have manuals for use but 48% of anesthesiologists do not read the manual and 60% do not follow a manufacturer’s checklist before using new equipment.26
Unfortunately, there are no laws or regulations regarding responsibility for assuring equipment use competency. There is one federal statue, the Safe Medical Devices Act of 1990, but this is directed toward the manufacturer and not the hospital or practitioner. Responsibility is diffusely disbursed among clinicians, facilities, and industry. The APSF has recently advocated mandatory training and certification before introduction of new critical care equipment into patient care.
Is Conventional Training Enough?
Dalley et al showed that after a standard introduction to complex and unfamiliar anesthesia equipment, clinicians were unable to self-assess their competence to use that equipment. They concluded that providers were likely to make multiple errors, which interacting with latent design faults may produce critical incidents.27 Similar findings were seen by Larson et al.28 Their study was performed during a nationally attended anesthesia meeting held at a large academic medical center. Anesthesia providers were observed performing anesthesia machine checkouts on an anesthesia machine with five preset faults. Regardless of experience, most anesthesia providers were unable to uncover a majority of the machine faults (Table 1–5).
In another recent study, eight simulated scenarios were developed, which included equipment failures or misuse.29 Second to fourth year residents completed the scenarios. A four item scoring checklist for each scenario was employed to evaluate completed items. Performance increased with experience but no perfect scores were obtained as shown in Table 1–6 Management of some types of machine problems may require annual review to assure continued competency. A simulation-based training environment may be helpful to develop and maintain these skills.
Table 1–5 Detection of Equipment Failure by Anesthesia Providers
How Should We Introduce New Equipment into the Operating Room and Train Anesthesia Personnel?
An APSF Board of Director’s Workshop was convened in 2007 to discuss the attitudes, evidence, comparisons, and recommendations for training on the use of complicated new equipment.26 Seventy-two participants from medicine, nursing, technical, administrative, regulatory, insurance, governmental, aviation, and safety industries participated (Table 1–7).
Table 1–6 Detection of Equipment Failure by Anesthesia Residents - by Year of Training
2007;104(1):154.28
Waldrop W, Murray D, Kras J. Simulation training for anesthesia equipment failure. Anesthesiology 2007;107:A1110.29
Table 1–7 Goals, Findings, and Recommendations of the APSF Workshop
Goals and Findings of the APSF Workshop
▸ THE PROBLEMS AND SHORTCOMINGS OF CONVENTIONAL TRAINING
Conventional “in-service” programs are thought to be superficial and inadequate. They usually do not require advanced preparation, are not mandated, do not allow individual practice, and do not test for learning or application skills. They are frequently abandoned for lack of time. These programs typically occur only once when new equipment is installed and do not account for personnel who are away from work or new personnel.30
▸ LIMITATIONS AND IMPEDIMENTS OF MANDATORY TRAINING
There are no published trials of mandated vs nonmandated training. Most believe that it would greatly benefit the specialty, but there is a need to establish baseline practices and to convince the staff that this is necessary and valuable. The most difficult obstacle is to figure out how to mandate the program to so many different categories of clinician
▸ DESCRIBE NEW APPROACHES TO TRAINING THAT MIGHT BE MORE SUCCESSFUL
Focus on new technology. Development of an in-house training program would be beneficial (train users and then have them train others, thus demonstrating understanding, and create superusers as resource personnel). Simulation and hands-on training show greater promise.
▸ CONSIDER ANALOGOUS END POINTS AND SUCCESSES FROM AVIATION MODEL
Aviation safety is regulated by the airline industry. As pilots die of their own deficiencies, they too support the efforts of regulating training. Mandatory retraining is derived from actual complications encountered in the previous year. If possible e-learning is also used in the aviation industry.
▸ EXPLORE THE REGULATORY AND THE MEDICOLEGAL AND REGULATORY PRESSURES DRIVING SUCH EFFORTS
Most believe that the Joint Commission would be the most appropriate regulatory body to oversee training for advanced medical devices. This training should be further tied in to the credentialing process and not be optional.
▸ PROMOTE DISCUSSION AND TARGET EFFORTS AT IMPLEMENTATION
Most believe that we should require mandatory training on all new equipment, but keep it focused on the critical aspects. To change culture and increase competency, a sense of accountability and responsibility needs to be instilled in the practitioners. We need to partner with other bodies, such as NPSF, Joint Commission, NQF, IHI, CMS, and insurance companies, to implement this training.
APSF Recommendations
Although existing literature does not describe frequent adverse anesthesia events owing to the anesthesia professional’s lack of understanding of equipment, the APSF believes that the logic is compelling to require confirmation of competency before using unfamiliar and/or complex anesthesia equipment that can directly affect patient safety. In this regard, the APSF believes that each facility should develop a required, formal process to ensure that anesthesia professionals have received appropriate training and/or demonstrated competence in the use of such medical devices.
Manufacturers should refine and initially offer this training. This required process for administering training and/or demonstrating competence should be efficient, timely, and pertinent in addressing new critical features and relevant failure modes. The most effective manner to successfully accomplish this training and testing is not known and requires deliberate investigation.
Larson E, Nuttall G, Ogren B, et al. A prospective study on anesthesia machine fault identification. Anesthesia Analgesia
SIMULATION AS A METHOD TO TRAIN FOR EQUIPMENT COMPETENCY
Simulation is used in most industries that handle hazardous materials, involve risk of injury, and face uncommon critical situations or in which operational errors have high costs. Thus simulation training and testing is ubiquitous in nuclear power, process control, aviation, military, and maritime industries. Simulation can be defined as a situation or environment created to allow persons to experience a representation of a real event for the purpose of practice, learning, evaluation, testing, or to gain an understanding of systems and human factors. The first medical simulation mannequin, Sim One, was developed in 1960 by Dr. Stephen Abrahamson at the University of Southern California. It was not until the 1980s that computer-controlled mannequins were created for anesthesiology training at two separate university centers: at the University of Florida at Gainesville by Drs. Nik Gravenstein and Mike Good and at Stanford University by Dr. David Gaba. These early innovators spawned a robust industry that now has international implications across most medical, nursing, and ancillary care disciplines. The advantages of simulation for medical training are obvious. It is possible to train in a totally safe environment where mistakes are not costly to real patients, to observe and evaluate performance, to create a reliable curriculum and to train teams in emergency management, communication skills, and especially the use of new equipment (Figure 1–2). The simulation lab can become an operating room (OR), intensive care unit (ICU), bed on a floor, or an emergency department (ED) bay (Figure 1–3). With the appropriate props, all clinical scenarios can be simulated. Although there are not yet rigorous studies showing that simulation training leads to better patient outcomes, anecdotal evidence and face validity have moved the field forward. Students enjoy experiential learning and there is good evidence that simulation learning is more profound than passive learning (e.g., lectures) (Figure 1–4). The biggest impediment to simulation is that it is very costly to build, outfit, staff, and maintain simulation facilities. Beyond training, simulation can be used to evaluate skills and behaviors, credential personnel, and evaluate equipment. Simulation is now a part of the Israeli national board examination for all anesthesiologists.30
The ABA has introduced simulation as a method to demonstrate maintenance of certification (MOCA) and the ASA has begun to endorse simulation centers for delivery of high quality simulation training (Figure 1–4).31 It seems inevitable that full-scale simulation will be an integral part of all anesthesiologists’ training, certification, and MOCA in the near future.
USE OF SIMULATION FOR DEVICE DESIGN AND USABILITY TESTING
Simulation for device design and usability testing is a more recent development. A few prospective trials have demonstrated the value of simulation in medical device design and evaluation. Such usability studies are carried out to ensure safe use of anesthesiology equipment in the simulated clinical environment.32 Kushniruk et al have made use of simulation to test out the effectiveness of new healthcare information systems and medication ordering systems.33 In another study, a new infusion pump was tested in simulation by 13 nurses during three scenarios. As a result of observations made during the simulation, changes were made to the hardware and software program, making it safer for patient use.34 The FDA has asked that companies seeking premarket clearance for clinical use of their products add to their application proof that the device can be used safely by typical users working under the normal range of conditions. Most manufacturers do usability studies in their own centers and then in clinical trials. Not all
Figure 1–3 Residents responding to a critical event in a simulation.
Figure 1–2 Team debriefing a simulation event.
Figure 1–4 Multidisciplinary team training.
clinical scenarios can arise during this testing, however. Using simulation to test devices under stressful conditions is very useful and centers such as the Center for Medical Simulation in Boston have worked with device manufacturers on just such studies.31
Dalley et al introduced a new anesthesiology delivery system with a new circuit design, fresh gas flow delivery, and ventilator control to 15 anesthesiology residents.27 In a randomized, controlled, prospective study they investigated the value of the addition of a simulator session compared with a traditional in-service one. Each group was tested in a second simulation involving an emergency situation. The group who had used the new anesthesia machine in a simulation scenario solved the emergency problem in less time and made fewer errors. Both groups made serious mistakes despite assessing themselves as competent in using the new machine.
Simulation was also used to evaluate the prototype of a new system for administering anesthesiology medications.35 The new system included special trays of medications with prefilled syringes, which were color- and bar-coded. Before administering the drug, the barcode would be read and the system audibly enunciated the name of the drug. Ten anesthesiologists performed simulated clinical scenarios with each system and then were asked to rate the new system for its acceptability, practicality, ease of use, propensity for error, and overall safety. Medication set up time was shorter with the new system. Three drug omissions occurred with the traditional system. With the new system, one event was prevented; the wrong drug was picked up for use but the auditory clue caused the provider to realize the error. The anesthetists believed the system was safe and clinically useful.
Simulation has also been used to study the use of anesthesiology monitoring. Lampotang et al demonstrated that the use of pulse oximetry and capnography shortened the time to detection and treatment of hypoxemia in a clinical scenario.36 This study is not ethical to do in human subjects since this type of monitoring is mandatory in the clinical setting. Overall there are many reasons why simulation can be useful to test medical devices but the most important one is that it does not impact patient safety.
In 2006 the APSF Committee on Technology launched an Anesthesia Workstation Training Initiative.37 The committee noted the increasing complexity of anesthesiology equipment and the paucity of literature on how best to train providers for safe and effective use. Technology training has been mandated by a few institutions in the United States and Europe. Olympio et al at Wake Forest University conducted a pilot project to train providers before introduction of a new and more complex electronic anesthesia workstation. Over a 2-month period, the learners had to complete four training components: attend a lecture, a hands-on workshop, a 30-minute simulation, and take a competency examination. Of 195 eligible participants, 54% completed the training. Most or many participants said that the hands-on and simulation components were the most valuable and endorsed more of these kinds of training experiences (especially if conducted close to the clinical areas). However, only 14% of faculty completed this training. Of note, while required by the department chair, residents and nurse anesthetists, the training was voluntary for faculty. This is unfortunate since faculty members are the ones who are called on should an equipment event occur. All authors emphasize the need for continual in-service for
new techniques and devices and a real appreciation for the inherent hazards of working with the unfamiliar.
Equipment safety in a complex system involving both humans and machines, human error is always a factor. What can manufacturers and clinicians use to improve equipment safety? A task checklist is one way to ensure systematic review of key portions of a task. Checklists can support teamwork and are authority neutral. The Food and Drug Administration and the ASA first endorsed a checklist for anesthesiology machine checkout in 1986. The FDA machine checklist has been revised and abbreviated but is still recommended.38 In another study, a checklist was developed for preoperative preparation for administering general anesthesia for a cesarean delivery.39 Twenty experienced anesthesiologists in a high-fidelity simulator prepared for induction both using and not using this checklist. On average, without the list, the participants missed one third of the items, but no items were missed when using the checklist. The simulation study made modifications to the checklist to make it easier to use in the clinical setting.
The ASA Committee on Standards and Practice parameters recently recommended that all practitioners receive training and demonstrate competence before use of any anesthesia workstation. They also recommended the use, the completion and documentation of a pre-use checkout before an anesthesia provider uses an anesthesia workstation on patients. The APSF has advocated that training be mandatory and implemented at the local level.
SIMULATION AS A QUALITY IMPROVEMENT TOOL
Simulation can be used to examine errors made in the clinical setting. During simulation scenarios, other problems may arise which are not anticipated. As reported by DeAnda,40 during a simulation session many other events can occur which are not expected. Analysis of these events can help to determine human errors or problems with equipment. Simulation has been used to observe the use of standard operating procedures (SOP).41Although the use of SOP is often evaluated by survey, one group of anesthesiologists was studied performing an SOP, rapid sequence induction, using simulation. They demonstrated more automatic functions than they described in the survey. The authors felt that using both a questionnaire and simulation could lead to better evaluation and improvement of SOP. Anderson et al studied the effect of using a disposable laryngoscope, which had been mandated for use by the United Kingdom Department of Health, to prevent prion infection.42 They demonstrated in simulation that the recommended disposable laryngoscope was more difficult to use than reusable ones, a problem not anticipated when the mandate was put into effect.
Conclusion
Anesthesiology is generally very safe, but poor patient outcomes still occur because of device failures and use errors. These are most commonly associated with equipment design flaws. An excellent understanding of all equipment (but especially new devices) is essential for safe patient care. The ASA and the APSF have advocated standardized, mandatory training and certification before the clinical use of new equipment
in the anesthesiology workstation. Simulation is an excellent tool for studying new devices and to train providers in its safe and effective use.
Further Reading
1. Cooper, J.B., Newbower, R.S., Kitz, R.J., 1984. An analysis of major errors and equipment failures in anesthesia management: considerations for prevention and detection. Anesthesiology 60 (1), 34–42.
2. Reason, J., 2005. Safety in the operating theatre — part 2: human error and organisational failure. Qual Saf Health Care 14 (1), 56–60.
3. Perrow, C., 1999. Normal accidents—living with high risk technologies. Princeton University Press, Princeton, NJ.
4. Wiklund, M.E., Wilcox, S.B. (Eds.), 2005. Designing usability into medical products. Taylor & Francis, Boca Raton, FL.
5. Sinz, E.H., 2007. Anesthesiology national CME program and ASA activities in simulation. Anesthesiol Clin 25, 209–223.
References
1. Kohn, L., Corrigan, J., Donaldson, M., 2000. To err is human: building a safer health system. Institute of Medicine, Washington, DC.
2. Clergue, F., 2008. What next targets for anaesthesia safety? Curr Opin Anesthesiol 2 (3), 360.
4. Lienhart, A., Auroy, Y., Péquignot, F., et al., 2006. Survey of anesthesiarelated mortality in France. Anesthesiology 105 (6), 1087–1097.
5. Arbous, M., Grobbee, D., van Kleef, J., et al., 2001. Mortality associated with anaesthesia: a qualitative analysis to identify risk factors. Anaesthesia 56 (12), 1141–1153.
6. Zhan, C., Miller, M., 2003. Excess length of stay, charges, and mortality attributable to medical injuries during hospitalization. JAMA 290, 1868–1874.
7. Lagasse, R.S., 2002. Anesthesia safety: model or myth? A review of the published literature and analysis of current original data. Anesthesiology 97 (6), 1609–1617.
8. Weinger, M., Slagle, J., 2002. Human factors research in anesthesia patient safety: techniques to elucidate factors affecting clinical task performance and decision making. J Am Med Inform Assoc 9 (6 suppl. 1), s58.
9. Oken, A., Rasmussen, M., Slagle, J., et al., 2007. A facilitated survey instrument captures significantly more anesthesia events than does traditional voluntary event reporting. Anesthesiology 107 (6), 909.
10. Cooper, J.B., Newbower, R.S., Kitz, R.J., 1984. An analysis of major errors and equipment failures in anesthesia management: considerations for prevention and detection. Anesthesiology 60 (1), 34–42.
11. Caplan, R., Vistica, M., Posner, K., et al., 1997. Adverse anesthetic outcomes arising from gas delivery equipment: a closed claims analysis. Anesthesiology 87 (4), 741–748.
12. Weinger, M.B., 1999. Anesthesia equipment and human error. J Clin Monit Comput 15 (5), 319–323.
13. Webb, R., Russell, W., Klepper, I., et al., 1993. Equipment failure: an analysis of 2000 incident reports: the Australian incident monitoring study. Anaesth Intensive Care 21 (5), 673–677.
14. Catchpole, K., Bell, M., Johnson, S., 2008. Safety in anaesthesia: a study of 12,606 reported incidents from the UK National Reporting and Learning System. Anaesthesia 63 (4), 340–346.
15. Posner, K. On behalf of the ASA closed claims project. May 18, 2009 (personal communication).
16. Eisenkraft, J., 2009. Hazards of the anesthesia workstation. ASA Refresher Courses in Anesthesiology 37 (1), 37.
17. Gilron, I., 1993. Anaesthesia equipment safety in Canada: the role of government regulation. Can J Anaesth 40 (10), 987–992.
18. Keresztury, M., Newman, A., Kode, A., et al., 2006. A surprising twist: an unusual failure of a Keyed filling device specific for a volatile inhaled anesthetic. Anesth Analg 103 (1), 124.
19. Harris, B., Weinger, M., 2006. An insidious failure of an oxygen analyzer. Anesthesia Analgesia 102 (5), 1468.
20. Gravenstein, D., Wilkhu, H., Liem, E., et al., 2007. Aestiva ventilation mode selector switch failures. Anesth Analg 104 (4), 860.
22. Reason, J., 2005. Safety in the operating theatre — part 2: human error and organisational failure. Qual Saf Health Care 14 (1), 56–60.
23. Weinger, M., Englund, C., 1990. Ergonomic and human factors affecting anesthetic vigilance and monitoring performance in the operating room environment. Anesthesiology 73 (5), 995–1021.
24. Perrow, C., 1999. Normal accidents: living with high-risk technologies. Princeton University Press, Princeton, NJ.
25. IEC 62366 ed 1.0, 2007 Medical devices- application of usability enginering to medical devices.
26. Olympio, M.A., 2008. Formal training and assessment before using advanced medical devices in the operating room. APSF Newsl 22 (4), 6–8.
27. Dalley, P., Robinson, B., Weller, J., et al., 2004. The use of high-fidelity human patient simulation and the introduction of new anesthesia delivery systems. Anesth Analg 99 (6), 1737–1741.
28. Larson, E., Nuttall, G., Ogren, B., et al., 2007. A prospective study on anesthesia machine fault identification. Anesth Analg 104 (1), 154.
29. Waldrop, W., Murray, D., Kras, J., 2007. Simulation training for anesthesia equipment failure. Anesthesiology 107, A1110.
30. Ziv, A., Rubin, O., Sidi, A., et al., 2007. Credentialing and certifying with simulation. Anesthesiol Clin 25 (2), 261–269.
31. Sinz, E., 2007. Anesthesiology national CME program and ASA activities in simulation. Anesthesiol Clin 25 (2), 209–223.
32. Wiklund, M.E., Wilcox, S.B. (Eds.), 2005. Designing usability into medical products. Taylor & Francis, Boca Raton, FL.
33. Kohn, L., Corrigan, J., Donaldson, M., 2000. To err is human: building a safer health system. Institute of Medicine, Washington, DC.
34. Lamsdale, A., Chisholm, S., Gagnon, R., et al., 2005. A usability evaluation of an infusion pump by nurses using a patient simulator. Proceedings of the Human Factors and Ergonomics Society 49th annual meeting.
35. Merry, A.F., Webster, C.S., Weller, J., et al., 2002. Evaluation in an anaesthetic simulator of a prototype of a new drug administration system designed to reduce error. Anaesthesia 57, 256–263.
36. Lampotang, S., Gravenstein, J.S., Euliano, T.Y., et al., 1998. Influence of pulse oxymetry and capnography on time to diagnosis of critical incidents in anesthesia: a pilot study using a full-scale patient simulator. J Clin Monit 14, 313–321.
37. Olympio, M.A., 2006. A report on the training inititaive of the committee of technology. APSF Newsletter 21 (3), 43–47.
38. U.S. Food and Drug Administration. Anesthesia apparatus checkout recommendations (website): http://www.asahq.org/clinical/fda.htm. Accessed July 2010.
39. Hart, E.M., Owen, H., 2005. Errors and omissions in anesthesia: a pilot study using a pilot’s checklist. Anesth Analg Jul 101 (1), 246–250.
40. DeAnda, A., Gaba, D.M., 1990. Unplanned incidents during comprehensive simulation. Anesth Analg 71, 77–82.
41. Zaustig, Y.A., Bayer, Y., Hacke, N., et al., 2007. Simulation as an additional tool for investigating the performance of standard operating procedures in anaesthesia. Br J Anaesth 99 (5), 673–678.
42. Anderson, K., Gambhir, S., Glavin, R., et al., 2006. The use of an anaesthetic simulator to assess single-use laryngoscopy equipment. Int J Qual Health Care 18, 17–22.
Medical Gases: Properties, Supply, and Removal
Jesse M. Ehrenfeld and Michael Oleyar
CHAPTER OUTLINE
Physical Principles of Medical Gases 10
Common Gas Laws 10
Critical Pressure and Critical Temperature 11
Medical Gases 11
Oxygen 11
Nitrous Oxide 12
Medical Air 13
Entonox 13
Nitric Oxide 14
Heliox 14
Xenon 14
In the United States, the supply and sale of medical gases and medical gas delivery systems are regulated by the Food and Drug Administration (FDA). Most other industrialized nations also regulate gases used for medicinal purposes— including Canada (by Health and Welfare Canada), the United Kingdom (by the Medicines and Healthcare products Regulatory Agency), and the European Union.1 Requirements for the manufacturing, labeling, filling, transportation, storage, handling, and maintenance of cylinders and containers for the storage of medical gases have been published by the U.S. Department of Transportation. The Department of Labor and the Occupational Safety and Health Administration (OSHA) regulates matters affecting safety and health of employees in all industries, including employee safety when dealing with waste anesthetic gases.2 Other safety measures, either voluntary or regulated, are published by The National Fire Protection Association (NFPA),3 the Compressed Gas Association (CGA), Canadian Standards Association (CSA), and the International Standards Organization (ISO). While regulatory measures are designed to ensure the safe and consistent manufacturing and use of medical gases, occasional accidents have been reported during their delivery.4 Unfortunately, these incidents have the potential to harm both patients and health care providers alike, especially anesthesiologists. Therefore proper precautions should be taken and backup systems must be put into place to minimize the impact of an adverse event. While regulatory measures play a large part in ensuring the safety supply of medical gases, perhaps even more important is the vigilant
Medical Gas Supply and Storage 15
Medical Gas Cylinder Safety 17
Medical Gas Pipeline Network and Manifold 18
Medical Gas Delivery to the Anesthesia Machine 19
Medical Gas Removal and Waste Gas 20
Vacuum 20
Scavenging Systems 21
Conclusion 21
anesthesia provider, who should always be mindful of medical gases and their safe delivery.
Physical principles of medical gases must be considered by anesthesia providers as each gas has its own unique properties, which can affect storage, delivery, and use. Medical gases may be found throughout the hospital, especially in anesthetizing locations such as operating rooms. Anesthesia providers must be aware of the sources of medical grade gas to ensure an adequate supply when delivering to patients. The three most common medical grade gases (oxygen, nitrous oxide, and air) are typically supplied via a large central source. Alternatively, these and other gases may be supplied via gas cylinders, most often “E” size cylinders mounted on the anesthesia machine. A waste anesthetic gas (WAG) scavenging system and a medical suction system for surgical and anesthetic use are also provided centrally.
Physical Principles of Medical Gases
Common Gas Laws
Medical gases may be stored either as liquefied gases (oxygen, nitrous oxide, carbon dioxide) or compressed gases (oxygen, air). The state in which a gas may be stored is dependent on the physical properties, and the relationship between pressure, volume, and temperature. These relationships are described by the Common Gas Laws (Table 2–1), also see chapter 27. Although the SI unit of pressure is the pascal (see Appendix I), anesthesiologists often measure and report pressure as kilopascals (kPa),
Table 2–1 Common Gas Laws
Gas Law Formula Relationship
Boyle’s law P1V1 = P2V2
Pressure and volume
Charles’ law V1/T1 = V2/T2 Volume and temperature
Gay-Lussac’s law P1/T1 = P2/T2 Temperature and pressure
Ideal gas law PV = nRT
Pressure, volume, and temperature
P, pressure; V, volume; T, temperature; n, number of moles; R, universal gas constant.
centimeters of water (cm H2O), pounds per square inch (psi), or millimeters of mercury (mm Hg). 1 kPa = 7.5 mm Hg and 1 mm Hg = 1.35 cm H2O.
Critical Pressure and Critical Temperature
Critical pressure and critical temperature are two important concepts that impact how medical gases are stored (also see, chapter 27). The critical temperature of a gas is defined as the temperature above which a particular gas is unable to be liquefied through the application of pressure. The critical pressure of a gas is defined as the pressure where a gas is able to be liquefied at the critical temperature of that particular gas.
The critical temperature of oxygen is −118° C, and, therefore, oxygen exists as a compressed gas at room temperature.5 The critical temperature of nitrous oxide is 36.5° C and therefore at room temperature nitrous oxide exists as a liquefied gas. To create liquid oxygen, one must first cool the oxygen below its critical temperature and then pressurize it. To maintain oxygen as a liquid, highly specialized containers, which are insulated and refrigerated, must be used.
Medical Gases
Oxygen
Medical grade oxygen is at least 99% pure. The commercial synthesis of oxygen begins with the liquefaction of compressed air. Factional distillation is then used to separate oxygen from liquid air by taking advantage of the differences in the boiling points of oxygen and nitrogen. During this process, nitrogen evaporates, first leaving liquid oxygen behind, which can then be evaporated and collected. In the medical environment, oxygen can be supplied as either a compressed gas from room temperature cylinders or as liquid oxygen from a cryogenic liquid system (CLS) container (Figure 2–1). Although the systems required to supply and store liquid oxygen are more involved and expensive than regular oxygen cylinders, they often are more economical in facilities in which higher volumes of oxygen are used. This is because liquid storage is less bulky and less costly than the equivalent capacity of high pressure gaseous storage.
Liquid Oxygen Storage Systems
A typical cryogenic liquid oxygen storage system consists of one or more cryogenic storage tanks (Figure 2–1), one or more vaporizers, a pressure control system, and the piping necessary
to support all of the requisite fill, vaporization, and supply functions (Figure 2–2). As previously mentioned, liquid oxygen must remain below its critical temperature of −118° F and pressurized to remain as a liquid. Because the temperature gradient between liquid oxygen and the surrounding environment is significant, keeping liquid oxygen well insulated from the surrounding heat is critically important.5 To accomplish this, cryogenic tanks are constructed in principle like a thermos bottle to shield the inner vessel from ambient heat. Vaporizers attached to the system convert liquid oxygen into a gaseous state. Downstream, a pressure control manifold then adjusts the gas pressure that is provided to the outgoing pipelines. Most cryogenic liquid systems include a backup system, which often consists of another smaller sized liquid oxygen container or a separate manifold of oxygen cylinders. In the event of an oxygen supply failure, anesthesia providers may be required to play a critical role in ensuring patient safety.6
Oxygen Concentrators
Oxygen concentrators, which can generate pure oxygen from atmospheric air, are occasionally used as the primary oxygen source in some remote locations. These devices work through the adsorption of atmospheric nitrogen by a molecule sieve. To ensure adequate delivery of oxygen, the oxygen output concentration should be carefully monitored when these devices are used. Most include a pressurized reservoir, which allows a stable supply of oxygen, even when demand on the system peaks. The size of the adsorption bed is the main determinant of the maximum output of the device, and a number of different sized oxygen concentrators are commercially available. Remote military bases may use large industrialsized units, which can supply enough oxygen for an operating room, whereas smaller portable units (Figure 2–3) are often used to provide home oxygen therapy.
The Dangers of Oxygen
Although oxygen is essential for life, the anesthesia provider must be aware of potential toxicities of oxygen. Exposure to high fractional concentration of oxygen for an extended time
Figure 2–1 A twin vessel cryogenic liquid system (CLS) installation.
may lead to lung injury via hyperoxia-induced necrosis and apoptosis.7 Oxygen is also extremely flammable and can act as an oxidizer for operating room fires.8 A change in oxygen concentration from 21% to 23% poses a fire threat even for substances that are not normally considered flammable, which reinforces the importance of adequate ventilation.5
Liquid oxygen can be splashed on items such as clothing, making them a fire hazard. Liquid oxygen may also cause freeze burns on direct contact, as it is −118° C. Nevertheless, oxygen is an essential drug used by every anesthesiologist. The physical properties of oxygen are summarized in Table 2–2
Nitrous Oxide
Joseph Priestley first described nitrous oxide preparation in 1772, and it is still prepared using the same principles.9 Nitrous oxide is synthesized commercially by heating ammonium nitrate and separating nitrous oxide from other compounds such as nitric oxide. Nitrous oxide exists primarily in the liquid phase at room temperature, and it is typically stored in large H-cylinders, which are cross-connected via an auto-switching manifold. Nitrous oxide storage banks typically have a smaller number of cylinders compared with oxygen supply systems because of the lower overall consumption of nitrous oxide and the higher content of liquefied gas. Nitrous oxide is often also found on the anesthesia machine in smaller E-cylinders. Because pressure within a storage cylinder remains stable around 745 psig at constant temperatures, in order to determine the amount of nitrous oxide left inside a given cylinder the cylinder must be weighed and compared against its tare weight.10 As liquid nitrous oxide is removed from a cylinder and approaches its critical temperature of 36.5° C,11 nitrous oxide reverts to the gaseous form. As nitrous oxide vaporizes from liquid to gas, heat is absorbed from the surrounding environment, which can lead to the formation of frost on the outside of the metallic gas cylinder.
Nitrous oxide has been used in clinical anesthesia for more than 150 years, although in the last 50 years there has been increasing concern over potential toxicity to patients, anesthesia providers, and the environment.12 Some choose to avoid nitrous oxide in patient care over concerns of postoperative nausea and vomiting, infection, pulmonary complications, endothelial dysfunction, and air-space expansion.13
C11
Figure 2–2 A simplified schematic of a single vessel cryogenic liquid system.
Figure 2–3 The Millenium Respironics Oxygen Concentrator—a portable oxygen concentrator for home use. (Image courtesy Respironics, Inc. and its affiliates, Murrysville, Pa.)
Table 2–2 Physical Properties of Common Pressurized Gases
Other data
Gas/vapor heavier than air. May accumulate in confined spaces, particularly at or below ground level
Gas/vapor heavier than air. May accumulate in confined spaces, particularly at or below ground level
Gas/vapor heavier than air. May accumulate in confined spaces, particularly at or below ground level. Note that due to its density, Xe will flow through standard flowmeters more slowly. A conversion factor of 0.468 should be applied
Currently supplied in N2 at less than 1000 ppm
Currently only supplied at mixtures of less than 0.3% in air and helium
Use the appropriate conversion chart when using oxygen flowmeters
N/A, Not applicable.
Note: all values STP.
Mixed gases, e.g., Heliox, will have different physical properties. For exact values, please contact the manufacturer. *Sublimation
Other concerns relate to the potential occupational hazards to anesthesia providers—because levels in operating rooms have occasionally been found to measure above national guidelines.14 Main occupational concerns include vitamin B12 depression, genetic damage, and reproductive compromise,15 although there is no definitive evidence for any of these side effects among hospital workers or anesthesiologists. Finally, nitrous oxide is also known to be a potent greenhouse gas, and some have discussed the possibility of nitrous oxide contributing to global warming.16 The physical properties of nitrous oxide are summarized in Table 2–2
Medical Air
Air is a nonflammable, colorless, odorless gas that makes up the natural atmosphere of the earth. Air consists primarily of nitrogen, oxygen, and water vapor—with small amounts of carbon dioxide and other elements mixed in. Medical air may be supplied from either a central plant or a series of cylinders connected
by an autoswitching manifold. Hospitals with a central plant typically have two compressors to ensure that the supply is not interrupted during service or maintenance. Having two compressors also allows the system to handle periods of peak demand. An air intake first brings air through a series of filters into a compressor. Most systems include an air cooler to cool the compressed air. From the compressor, air then passes through a one-way valve into a reservoir, where a constant pressure is maintained. Upon leaving the reservoir, another series of filters and separators removes particulate matter and impurities such as oil droplets from the pressurized air supply. A set of driers containing chemical desiccant then eliminates excess humidity from the air and a final bacterial filter removes any contaminants.
Entonox
Entonox is a 50:50 mixture of nitrous oxide and oxygen that is stored at a pressure of around 2000 psig in metallic cylinders. While used mostly for dental and obstetric analgesia,
changes,17 and transport of patients with long bone fractures, some recent studies have suggested broadening its use for procedures such as bone marrow biopsy18 and extracorporeal shock wave lithtripsy.19 Entonox is typically self-administered to patients while under medical supervision via a two-stage pressure regulator that is connected to a demand valve. Because Entonox is a mixture of gases, although the critical temperature of nitrous oxide is 36.5° C, nitrous oxide usually remains in gaseous phase. At temperatures below −5.5° C (the pseudocritical temperature), it is possible for a liquid phase to form below the gas containing 80% nitrous oxide and 20% oxygen. Because of this phenomenon, Entonox cylinders are typically stored in environments where the temperature is greater than 10° C to prevent the administration of a hypoxic mixture of gases.
Anesthesiologists should be aware of the potential for spreading infectious and/or communicable diseases via the Entonox apparatus should it be connected to the anesthesia machine. Because of the potential for cross-infection, a new Entonox apparatus must be used with each patient or a special antimicrobial filter must be placed on the tubing.20
Nitric Oxide
Nitric oxide, a poisonous gas whose clinical utility has only recently been established, has found increasing use for treatment of pulmonary dysfunction in the intensive care unit and operating room and is currently being studied as a possible therapeutic agent for treatment of acute myocardial infarction.
Inhaled nitric oxide (iNO) has a profound impact on pulmonary vascular tone. Although the only current Food and Drug Administration–approved use of iNO is the treatment of hypoxic respiratory failure due to pulmonary hypertension in neonates, inhaled nitric oxide is occasionally used to treat pulmonary hypertension in adults.21
Nitric oxide is stored in a nonliquefied form as a gaseous blend of nitric oxide (800 ppm) and nitrogen at a cylinder pressure of 2000 psig at 21° C. Administration occurs by injecting nitric oxide into a ventilator breathing circuit via a monitoring unit (NOx Box, Nodomo unit, iNOvent), which operates at 55 to 60 psig. The usual initial dose of nitric oxide is 5 to 20 ppm. Monitoring the levels of inspired nitric oxide, nitrogen dioxide levels, and methemoglobin levels is essential because methemoglobinemia is a well-known toxicity of iNO use.22 Patients should be slowly weaned off inhaled nitric oxide in decrements of 5 ppm over 6 to 8 hours and under no circumstances should nitric oxide ever be abruptly discontinued. The physical properties of nitric oxide are summarized in Table 2–2
Heliox
Helium-oxygen mixtures (heliox) have been used for medicinal purposes for almost 100 years. Helium is an inert gas, which also has no odor, color, or taste and does not support combustion or react with biological membranes.23 Helium is 86% less dense (0.179 g/L) than room air (1.293 g/L). Administration may be conducted using either an endotracheal tube or face-mask with a reservoir bag.
Treatment with heliox takes advantage of the low density of helium to improve respiratory mechanics in a number of
pathological states by reducing the Reynolds number associated with flow through the airways, thereby increasing laminar flow and breathing efficiency. This technique is particularly effective in obstructive conditions.24 In addition to increasing laminar flow, heliox also facilitates gas diffusion, which may enhance alveolar ventilation in some circumstances.23 Heliox has been studied and reported by some to be effective in a variety of respiratory conditions, such as upper airway obstruction, status asthmaticus, decompression sickness, postextubation stridor, bronchiolitis, ARDS, and COPD,25 although more recent studies failed to show an improvement in reintubation rates when using heliox.26 It is important to note that heliox mixtures have the potential to decrease the work of breathing in patients with increased airway resistance, but administration does not “treat” airway resistance and is not a substitution for alleviating obstruction.
Heliox is supplied in compressed medical gas cylinders, which are typically provided in sizes E, H, and G. Helium and oxygen are usually blended to provide concentrations of 80/20, 70/30, or 60/40. Gas regulators manufactured and designed specifically for the delivery of helium must be used to deliver the gas accurately and safely. The high costs and technical challenges of delivering heliox, combined with a lack of overwhelming evidence demonstrating clinical benefit, has limited everyday use of heliox.27 The physical properties of heliox are summarized in Table 2–2
Xenon
Xenon is a noble gas, sharing nontoxic and physiologically inert properties with helium. It is found in the atmosphere in a concentration of 0.00005 ppm (50 cubic meters of air contains 4 mL of xenon).28 Xenon is a more potent anesthetic gas than nitrous oxide with a minimum alveolar concentration (MAC) of approximately 71%.29 It is poorly soluble in the blood with a blood/gas distribution coefficient of 0.115, which means it has rapid onset and offset. Administration is similar to other inhaled anesthetics, but special regulators are required to accurately determine administration values, as xenon gas is approximately five times denser than air.
Xenon is a gas still in the experimental phases for medical use. Xenon has anesthetic properties, which have been known since 1939 but were not reported until 1946 when Lawrence published a paper on xenon anesthesia in mice. Since then, multiple reports of successful use as an anesthetic have been published, and many benefits over nitrous oxide have been described such as better circulatory stability, better end-organ perfusion, and reduced neurocognitive dysfunction after general anesthesia.30 Environmental advantages of its use have also been documented. Unlike the fluorinated hydrocarbons in use, xenon does not deplete the ozone layer.
Xenon has advantages as a noble gas anesthetic, but still is not a major part of the anesthesiologist’s armamentarium. A major prohibitive factor is the high cost associated with a complicated production and delivery process. Current cost estimates show xenon anesthesia as costing roughly 10 times as much as conventional anesthesia. Closed circuit apparatuses with unique gas detector systems have been designed as a way to recycle gas and reduce costs and may help lead to greater use of xenon as an anesthetic in the future. 31,32 The physical properties of Xenon are summarized in Table 2–2 .
Medical Gas Supply and Storage
Although medical gases each have unique properties, universal safety measures apply to their supply and storage. Medical gases are stored in different types of cylinders, ranging in size from 1.2 L to 7900 L, which are fitted with different types of valves.33 Cylinders are mostly made of steel alloys or aluminum. Before being placed into use, each medical gas cylinder is tested by visual inspection and with a hydraulic stretch test to ensure that the vessel will maintain its integrity when subjected to test pressures that reach at least 1.66 times the rated service pressure. Once tested, each medical gas cylinder is permanently stamped with a symbol that indicates its contents, service pressure, manufacturer’s symbol, serial number, owner’s symbol, test date, and testing facility. After a cylinder has been tested and filled, a cylinder label and a separate batch label (Figures 2–4 and 2–5) will be affixed indicating the contents of the cylinder, directions for use, batch number, fill, and expiration date. This information is important, should the cylinder be involved in a recall or accident.
Table 2–3 summarizes the color codes, state in cylinders, and pressure for different medical gases.34 It is important to remember that cylinders may contain contents under high pressures, and safety measures should always be taken to prevent catastrophe. Extreme temperatures and rapid temperature changes must also be avoided to prevent cylinder damage and/or leakage.
Valves: Common valves types include pin index valves (Figure 2–6), bullnose valves (Figure 2–7), hand-wheel valves (Figure 2–8), and integral valves.33 The valve types on any given cylinder will typically vary according to the cylinder size. Smaller cylinders (Type E cylinders), which are commonly used as backup reservoirs on an anesthesia machine, usually have pin indexed valves (Figure 2–6). Large bulk cylinders, which supply hospital pipelines (Type H cylinders), are usually fitted with bullnose valves (Figure 2–7). The bullnose valve includes a noninterchangeable screw thread system, with a different number of threads per inch for each medical gas.
Opening/closing cylinders: Small cylinders (Type E) usually have a spindle, which may be opened or closed with a wrench. The correct-sized wrench is usually permanently attached to the back of the anesthesia machine to prevent removal. Larger cylinders (Type H) include a hand-wheel
valve (Figure 2–8), which may be used to open or close the valves without any additional equipment.
Cylinder filling: Medical gas cylinders should not be overfilled—because of the risk of accidental explosion and/or damage with overpressurization. It is important to keep in mind that the pressure inside a cylinder will vary with the ambient temperature—and a cylinder filled to a given pressure at one temperature might not withstand the pressure changes associated with a drastic increase in ambient temperature. Each cylinder will have a service pressure stamped on it. To prevent damage or possible explosion, cylinders should never be filled above the service pressure. For cylinders that contain liquefied gases, the filling limit is based on the filling density or percent ratio of weight of a gas in the cylinder to the weight of the water in the cylinder. For both nitrous oxide and carbon dioxide, the filling density is 68%.
Safety pressure release valves: All medical gas cylinders are fitted with a safety pressure release valve, which allows the gas inside to escape, should the internal pressure rise too high. Three types of safety release valves are commonly used; spring-loaded pressure relief valves, discs that rupture at a predefined pressure, or metallic plugs that melt at high temperatures. Note: on cylinders with pin-indexed valves, the safety pressure release is located directly below the conical depression for the screw clamp. Care should therefore be taken not to damage this valve while mounting cylinders with pin-indexed valves onto the back of the anesthesia machine.
Calculating a cylinder’s contents: It is possible to determine the contents of a medical gas cylinder that contains a compressed gas (such as oxygen or air) using Boyle’s law (P1V1 = P2V2) where P1 stands for the atmospheric pressure, V1 the volume of gas at atmospheric pressure, P2 the pressure in the cylinder, and V2 the water capacity of the gas in compressed state (see also Common Gas Laws section). Therefore the pressure in an oxygen or air cylinder (which can be directly measured by use of a
Figure 2–4 Cylinder label.
Figure 2–5 Batch label.
Table 2–3 Medical Gas Cylinders34
Oxygen Air
Entonox
Nitrous oxide
Figure 2–6 Pin index safety system for different gases. Two pins in the hanger yoke of anesthesia machine are aligned with two corresponding holes on the cylinder head to prevent mounting of wrong cylinder.
Figure 2–7 Bullnose cylinder valve.
Figure 2–8 Medical gas cylinder with hand-wheel valve.
Table 2–4 Relative Sizes and Specifications of Commonly Used Oxygen Cylinders
Table 2–5 Relative Sizes and Specifications of Commonly Used Nitrous Oxide Cylinders
Table 2–6 Relative Sizes and Specifications Of Commonly Used Entonox Cylinders
Entonox, a mixture of 50% oxygen and 50% nitrous oxide, exists as a gas. The pseudocritical temperature of Entonox in pipelines at 4.1 bar is below –30° C. Nitrous oxide in an Entonox cylinder however begins to separate out from Entonox if the temperature falls below –6° C. A homogenous mixture is again obtained when the temperature is raised above 10° c and the cylinder is agitated.
pressure gauge) is directly proportional to the volume of gas remaining.
For liquefied gases, such as nitrous oxide and carbon dioxide, it is not possible to estimate the contents of the cylinder by measuring pressure because the pressure inside the cylinder will stay constant until all the liquid has evaporated. To determine how much gas remains in a cylinder of liquefied gas, one must weigh the cylinder. By subtracting the tare weight (the weight of an empty cylinder, which is permanently stamped on the outside) from the total cylinder weight, one can estimate the contents of liquefied gas remaining.
Temperature: Liquid oxygen stored in medical gas cylinders is −118° C and can cause immediate tissue damage on
contact. It is therefore important to avoid direct contact with liquefied gases stored in cylinders.
Cylinder sizes: Tables 2–4 to 2–8 give details for oxygen, nitrous oxide, Entonox, carbon dioxide, and heliox. The water capacity of the various cylinder sizes is given in Table 2–4
Medical Gas Cylinder Safety
Medical gas cylinders should always be properly stored and secured to prevent damage and/or injury. Cylinders should never be dropped because a cracked pressurized cylinder can turn into a high speed projectile. All cylinders should be stored
Table 2–7 Relative Sizes and Specifications of Commonly Used Carbon Dioxide Cylinders
Table 2–8 Relative Sizes and Specifications of Commonly Used Heliox21 Cylinders
Table 2–9 Factors Contributing to Pollution of the Operating Room
1. Use of breathing systems with high flow anesthetic techniques
2. Poorly fitting masks
3. Failure to turn off gases at the end of anesthetic
4. Filling anesthetic vaporizers without key systems
5. Volatile agent spills
6. Leaks in the anesthesia machine and/or breathing circuit
7. Ineffective waste anesthetic gas scavenging
All individuals who are expected to handle medical gas cylinders should receive training and education about their proper use and storage. Medical gas cylinders should (1) always be opened slowly to prevent rapid temperature rise from adiabatic expansion and (2) always be kept closed when not in use. Although medical gas cylinders are one of the simplest pieces of equipment used in the operating room, they have the potential to be one of the most dangerous if not used and handled properly.
Medical Gas Pipeline Network and Manifold
The main oxygen supply in the operating room, and for the anesthesia machine, is the hospital pipeline. This system delivers oxygen at 55 pounds per square inch gauge (psig) and comes from one of two sources:
1. A primary liquid oxygen tank (with either a manifold of compressed gas cylinders or a smaller secondary liquid oxygen tank as a backup)
2. Two redundant banks of compressed gas cylinders (with a smaller bank of compressed gas cylinders as a backup)
A hospital should always have at least a 2-day supply of oxygen on-hand, and a backup supply with at least a 1-day supply. The specific total amount required will, of course, depend on the particulars of a hospital, its patient care volume, and specific needs.
The high pressure oxygen source connects to the hospital pipeline through a two stage pressure regulator. In hospitals where a manifold of oxygen cylinders is used, only one of the two oxygen banks will supply the main pipeline at any time. Once the first bank becomes exhausted, the second bank will automatically switch over. This allows the depleted cylinders to be changed out, without having to
disrupt the whole system. The entire system is typically monitored by a centralized manned control station where visual indicators show the status of the two banks at all times.
All of the main hospital gas supplies and manifolds (Figure 2–9) are typically physically located outside of the hospital itself. A series of gas supply pipelines take gas from the manifold to the required delivery points around the hospital campus. Safety standards for both oxygen and other positivepressure medical gases require the use of copper tubing to prevent the spontaneous combustion of organic oils. The supply networks are designed with pressure monitors and shut off valves (Figure 2–10) throughout the system to allow isolation of problem areas for maintenance or emergency repairs. Supply lines typically have the contents and flow direction labeled at regular intervals.
Medical gas outlets: All hospital pipelines will end in terminal wall outlets. These color-coded outlets come in one of two varieties: (1) Diameter index safety system (DISS) outlets or (2) noninterchangeable quick coupling connectors.
DISS: The DISS was developed to prevent accidental wrong connections among different medical gases (Figures 2–11, 2–12). The connector system makes it physically impossible to connect the wrong hose to the wrong pipeline. Each connector is made up of a body, nipple, and nut. The body has two concentric bores, which match specific shoulders on the matching nipple. The diameters of the bores are different for each gas, making then noninterchangable. Only appropriately matched parts will fit together and allow a complete connection. Noninterchangable quick connectors: Quick connectors allow flow meters, hoses, machines, and other pieces of equipment to be quickly connected/disconnected without using any tools or large amounts of force. Each quick
connector is made up of a pair of gas-specific male and female pieces. The two components are then locked together by a releasable spring mechanism. Different shapes prevent hoses from being placed into the wrong outlet. While quick connectors may be easier to use than DISS connections, they tend to leak with a higher frequency.
Medical Gas Delivery to the Anesthesia Machine
Whether using medical gas from a central hospital supply or free standing cylinders, anesthetic gases are delivered in the operating room using an anesthesia machine. It is essential that the anesthesia machine be properly attached to its gas
Service connection 22 mm to distribution system
Spare rack
Figure 2–9
nitrous oxide cylinder manifold (A), with schematic shown below (B)
sources—because incorrect connections can lead to disaster. As discussed previously, the valves for each medical gas are unique and have been designed to prevent improper connections. However, the connections between the anesthesia machine and the wall supply should be checked daily to ensure there is no physical damage or disconnect. Additionally, the pressures in the anesthesia machine should be checked to ensure that there is an adequate quantity of medical gas
available.35 Vigilance must always be practiced, especially in unfamiliar anesthetic delivery areas, because wrong gases can be mistakenly delivered.36
Medical Gas Removal and Waste Gas
Medical gas removal is just as important as medical gas delivery. Anesthetic gases and the excess vapors that leak into the surrounding environment during surgical procedures are considered waste anesthetic gases. The number of health care professionals in the United States who are potentially exposed to waste anesthetic gases has been estimated to be in the range of 250,000 individuals annually.2 For a number of years, there have been questions about the potential relationship between exposure to trace concentrations of waste anesthetic gases and the possible development of adverse health effects. The potential effects of exposure to waste anesthetic gases include symptoms such as headaches, fatigue, nausea, dizziness, and irritability. Some also claim that exposure to waste anesthetic gas increases the risk of sterility and/or miscarriages among operating room personnel. However, the topic remains controversial and the evidence that trace anesthetic gases are harmful is at best suggestive, rather than conclusive.
It is nonetheless recommended to scavenge waste anesthetic gases to reduce the potential for excess contamination/ exposure. Medical gas removal is therefore an important consideration for the well-being of not just the patient, but also the patient’s healthcare providers. Design for removal systems is similar to design for delivery systems, and vacuum removal is an important component of the overall system design.
Vacuum
A vacuum is defined as a volume of space that has no matter in it. This results in a space having a much lower (negative) pressure than atmospheric pressure. Vacuums are important because they are used to create suction. Suction can then be used within the operating room to remove gases, liquids, and solid materials from both patients and the environment.
Hospitals typically provide a vacuum to each patient care location via a pipeline system, which is capable of delivering a vacuum of close to 300 mm Hg at each terminus. Vacuum pipelines are typically constructed the same way medical gas supply lines are (copper tubing) but are usually larger in diameter. A vacuum is created placing two pumps in parallel with a reservoir between them to (1) even out the vacuum and (2) remove any debris from the system. Filters, before and after the wall connection, are important to prevent major leakage of harmful or
Figure 2–10 A typical medical gas control valve, which may be used to isolate the medical gas supply to a specific operating room or group of operating rooms.
Figure 2–11 Medical gas hoses employing the diameter index safety system. Note that each color-coded hose has a different connection diameter, which matches the corresponding wall terminus (see Figure 2–12).
Figure 2–12 Terminal wall outlet. Note that the different diameters match the corresponding flexible gas hoses (Figure 2–11).
contaminated materials into the central vacuum system. A diagram of a typical hospital vacuum plant is shown in Figure 2–13
Scavenging Systems
A scavenging system is designed to vent excess anesthetic gas from the ventilator and breathing circuit. Most systems collect waste gas from across a ventilator’s relief valve. Waste anesthetic gas is then transferred via special tubing into the scavenging interface. These tubes are intentionally made into a different size and appearance, so that they cannot be accidentally connected to the breathing circuit. The scavenging interface prevents excessive positive or negative pressure from coming into contact with the breathing system. Most scavenging systems employ an active, central vacuum to remove excess gas—although some rely on the pressure of the waste gas itself to move excess anesthetic through the scavenging system, i.e. a passive system. Occlusion of a scavenging system can lead to excess positive pressure being transmitted into the breathing circuit and therefore must be avoided. All hospitals should have a standing program for management of waste anesthetic gases, including a documented maintenance schedule for the ventilation system in the operating room, postanesthesia care units, and the anesthesia machines (Table 2–9).
Conclusion
Medical gases are a critical element of the practice of anesthesiology, but also a potential source of harm for patients and anesthesia providers alike. It is therefore critical that
personnel who work in the operating room understand how to properly use, maintain, and store medical gases. Although the practice of anesthesiology is constantly changing, such as a recent trend toward increased use of total intravenous anesthetics (TIVA),14 our medical gases and their delivery will always be an important part of perioperative care.
Suggested Further Reading
American Society of Anesthesiologists. Waste anesthetic gases: information for management in anesthetizing areas and the postanesthesia care unit (PACU) (website): http://www.asahq.org/publicationsAndServices/wasteanes.pdf. Accessed October 5, 2009.
Davey, A., Diba, A., Ward, C.S. (Eds.), 2005. Ward’s anaesthetic equipment, ed 5. Elsevier Saunders, Philadelphia.
Eichhorn, J.H., 1981. Medical gas delivery systems. Int Anesthesiol Clin 19 (2), 1–26.
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Figure 2–13 Main components of a medical vacuum plant.
